BACKGROUND: Graft versus host disease (GvHD) is the main complication after hematopoietic stem-cell transplantation.Extracorporeal photochemotherapy (ECP) is a cell therapy currently used for the treatment of T-cell–mediated diseases and seems as a valuable second-line therapy for patients suffering from steroid-refractory acute or chronic GvHD. ECP induces the apoptosis of treated cells and is believed to elicit a specific immune regulation of alloreactive T cells through repeated apoptotic T-cell infusions. However, its mechanisms of action have not yet been elucidated. In GvHD,alloreactive but not non alloreactive T cells are continuously activated by their environment. We hypothesized that ECP has a differential apoptotic effect on activated compared with resting T cells.METHOD: The ECP-induced apoptosis of resting and activated T cells from patients with chronic GvHD was assessed.The kinetic of apoptosis was also evaluated using several triggers of T-cell activation such as mitogenic or antigen specific activation. The influence of survival cytokines (interleukin-2, -7, and -15) was also evaluated.RESULTS: Activated T cells from patients with chronic GvHD underwent apoptosis faster than resting T cells. This phenomenon was confirmed using mitogenic and antigen-specific activated T cells from healthy donors and cannot be delayed by protective cytokines.CONCLUSIONS: ECP induces a faster apoptosis of alloreactive activated T cells than of non alloreactive resting T cells in GvHD and more generally of activated T cells than of resting T cells. These novel findings provide new insights about the ECP-induced specific control of pathogenic T cells in GvHD.